1. Field of the Invention
The present invention relates generally to electrically active ceramic devices and, more particularly, to segmented flextensional piezoelectric devices arranged into a cooperative array.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field. Piezoelectric and electrostrictive devices (generally called "electroactive" devices herein) are commonly used as drivers, or "actuators", when subjected to an applied electric field, due to their propensity to deform under such electric fields. When used as an actuator, it is frequently desirable that the electroactive device be constructed so as to generate relatively large deformations and/or forces from the electrical input.
Various configurations of electroactive actuators have been proposed in the prior art in order to increase the amount of deformation and/or output force which can be generated by such devices. Direct mode actuators make direct use of a change in the dimensions of the ceramic material when activated, without amplification of the actual displacement, and are able to achieve a very small displacement (strain), which is, at best, only a few tenths of a percent. Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Prior flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, indirect mode actuators can exhibit several orders of magnitude greater displacement than can be produced by direct mode actuators.
The magnitude of the strain of indirect mode actuators can be increased by constructing them either as "unimorph" or "bimorph" flextensional actuators. A typical unimorph is a concave structure composed of a single piezoelectric element externally bonded to a flexible metal foil, and which results in axial buckling or deflection when electrically energized. Common unimorphs can exhibit a strain of as high as 10% but can only sustain loads which are less than one pound. A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surface of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than comparable unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20% (i.e. about twice that of unimorphs), but, like unimorphs, typically can only sustain loads which are less than one pound.
For certain applications of electroactive actuators known in the prior art, asymmetrically stress biased electroactive devices have been proposed in order to optimize the axial deformation of the electroactive material. In such devices, (which include, for example, "Rainbow" actuators (as disclosed in U.S. Pat. No. 5,471,721), "Thunder" actuators (as disclosed in U.S. Pat. No. 5,632,841), and other flextensional actuators) the asymmetric stress biasing produces a curved structure, typically having two major surfaces, one of which is concave and the other which is convex.
It is well known in the art that piezoelectric materials (such as PZT ceramics) are typically very brittle. When curvilinear piezoelectric elements made of such brittle materials are subjected to electrical energy, they tend to bend, and the convex surface of the element (i.e. at the "outside" of the bend) may undergo sufficient tension to cause the piezoelectric material to fracture. In most piezoelectric materials the achievable piezoelectric (i.e. output) force is proportional to the thickness of the element. However, because of the brittle nature of most PZT ceramics, only extremely thin (typically on the order of many thousandths of an inch thick) ceramics can, as a practical matter, be deformed into curved shapes; typically to relatively large radii of curvature, (or more particularly to radii of curvature which are necessarily a relatively large fraction of their unenergized radii of curvature); with relatively low piezoelectric output forces; and with relatively low total axial displacement.
Accordingly, it would be desirable to provide a curved electroactive ceramic device which, when electrically energized, is capable of large axial displacement, is capable of large radially directed output forces, and which is capable of deforming such that its energized radius of curvature is a relatively small fraction of the unenergized radius of curvature of the device.